Luminescent materials are widely used in light sources for converting light of a first color into light of a second color. Such use is discussed in U.S. Pat. No. 7,462,878. The luminescent material is arranged in a compact converter layer. The temperature rises in the converter layer because of the conversion of light with a high energy into light of a longer wavelength (Stokes shift), which has a lower energy. The difference of energy is dissipated and as such converted into heat. At the outside surface of the converter layer the heat is transferred to the ambient and/or to a heat sink. The luminescent material may be an organic or an inorganic luminescent material. A problem of organic luminescent materials is their photo-chemical stability, which strongly depends on the temperature of the material during illumination.
If an organic luminescent material is used in the converter layer of the cited patent, the lifetime of the luminescent material will be relatively short. The temperature of the converter layer becomes relatively high in operation and as such the quality of organic material degrades and after a relatively short period the conversion by the organic luminescent material will be less efficient and/or less light will be converted towards the second color.